Internet Security and Key Exchange Basics in .NET

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Internet Security and Key Exchange Basics
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The IKE specification [IKE2409] provides a faster alternative for phase 1, called aggressive mode, which requires only three messages (1.5 round trips) as opposed to the three round trips required by the main mode. As shown in Figure 4.6, aggressive mode combines the cookie and proposal exchange with the Diffie Hellman exchange. Also as one can see in the figure, the peer identities are carried in the clear, which means in the aggressive mode is less secure when it comes to protecting the privacy of the clients. IKE specification provides two modes for phase 2 as well. Besides the quick mode, there is another mode called new group mode, which can be used to change the cryptographic group for future negotiations without establishing any new SAs on its own.
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4.3.3 ISAKMP: The Backstage Protocol for IKE
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To go further into the details of the IKE, one needs to understand the details of ISAKMP better. ISAKMP is described in a fairly thorough manner in RFC 2408 [ISAKMP2408]. First, we can see the resemblance between the ISAKMP and IKE modes. The main mode in IKE phase 1 is an instantiation of the ISAKMP identity protect exchange, which consists of a policy exchange, followed by a Diffie Hellman exchange and an authentication exchange. The IKE aggressive mode is also an instantiation of the ISAKMP aggressive exchange. Understanding details of IKE messaging and especially the newer profiling work that is being done to fine-tune the certificates for IKE (as explained in 9) requires better understanding of ISAKMP message and payload formats.
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4.3.3.1 ISAKMP Message Format Each ISAKMP message has a header with a fixed format, followed by a number of ISAKMP payloads (Figure 4.7).
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(1) Cookie and proposals, DH exchange, Ci, ISAi, X, Ni, IDi (2) Cookie and proposal, DH exchange, authentication Cr, ISAr, Y, Nr, IDr, Authr Initiator (3) DH exchange, authentication Ci, Cr, Authi Responder
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IKE phase 1 conversations according to aggressive mode
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ISAKMP header
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ISAKMP payload 1 Figure 4.7
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ISAKMP payload N
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ISAKMP message format
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Initiator cookie (CKY_I): 8 octets (64 bits) Responder cookie (CKY_R): 8 octets (64 bits) Next payload Version no Exchange type flags
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Message ID Length Figure 4.8 ISAKMP header format
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The ISAKMP header, which is referred to as HDR in IKE documentation and in pictures in the rest of this chapter, is shown in Figure 4.8. The ISAKMP header is sent unencrypted even after ISAKMP SA (IKE SA) is established. As mentioned earlier, since ISAKMP SA is identified by the initiator and responder cookies, these cookies are included in the header for ISAKMP exchanges.
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The next payload field in the ISAKMP header indicates the type of the first payload included in the ISAKMP message. Table 4.3 shows some examples of the payloads that will be discussed in the following text. These payload types are also used for nesting of multiple payloads in a single ISAKMP message. The next payload field identifies the type of the following payload in the ISAKMP message and uses the types shown in Table 4.3. The field exchange type in the ISAKMP header indicates the type of exchange in progress. These exchange types translate into the IKE modes, such as main and aggressive modes (as explained earlier). Several ISAKMP exchanges types are defined such as, base , authentication only , aggressive , and so on.
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Each ISAKMP payload (all following the ISAKMP header in the message) begins with a generic payload header. The payload header has a simple format as shown in Figure 4.9.
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Table 4.3
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Examples of ISAKMP payload types Value 0 1 2 3 4 5 6 7 8 9 10
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Next payload type None Security association (SA) Proposal (P) Transform (T) Key exchange (KE) Identification (ID) Certification (CERT) Certificate request (CERTREQ) Hash (HASH) Signature (SIG) Nonce (NONCE)
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Internet Security and Key Exchange Basics
Next payload
Reserved
Payload length
Figure 4.9 ISAKMP generic payload header
Now that we explained the building blocks of the ISAKMP messages, let us go through the semantics of a few important payload types that are important for understanding the details of IKE messaging and authentication.
SA payload: This payload is used for negotiation of security attributes and to indicate the DOI that helps the two parties interpret the context of the other ISAKMP payloads. For instance, the DOI defines how the cryptographic algorithms and certificate authorities (CAs) should be named in a consistent manner or how a proposal for protection of traffic under a given situation is formatted. Proposal payload: This payload is used by each party to propose a security protocol for protection of the data. The protocols are identified by protocol identifiers and may include IPsec ESP, IPsec AH, and TLS depending on the type of protection needed for the data communications (authentication, encryption, or both). In order to facilitate the negotiation, each party may include multiple proposal payloads along with each exchange message. The proposal payload includes a proposal number that is used in a clever manner: when a party needs to use a bundle (multiple) of security protocols to protect the data, for instance both ESP and AH, then that party uses the same proposal number within the proposal payloads for each of those protocols. In other words, using the same proposal number in multiple payloads has a logical-AND meaning. The message to the other peer is I need to use ESP AND AH to protect the data . On the other hand, using different proposal numbers for each proposal has a logical-OR function and means I need to use either ESP or TLS . The proposal payload also includes a number of transform field and a sending entity SPI , whose size and type depends on the protocol that is being offered. Transform payload: Each proposal payload is a proposal to use a specific security protocol such as IPsec ESP. As we know, each security protocol can use a number of transforms (security algorithms). The transform payload is used within the security association negotiation exchange to propose a transform such as DES or SHA1. The transform payload includes a transform number field. The responder chooses a transform within those proposed in conjunction with each proposal number to indicate its choice of security algorithm. Key exchange (KE) payload: As mentioned in 3, to avoid overuse of the limited bandwidth resources, IKE facilitates the negotiation of Diffie Hellman (or Oakley) parameters between the two parties, without the need for transfer of the complete parameter data. IKE accomplishes this by using the ISAKMP KE payload defined in [ISAKMP2408]. This way only the group numbers, rather than the parameters that define the group, need to be sent. It should be noted that the actual group parameters are however defined by IKE specification [IKE2409] rather than by ISAKMP specification. Identification payload: This payload is used to provide the identity of the communicating peer. We will refer this payload again when describing the use of certificates for IKE in 9.